This systematic review of observational research in adults revealed that the association between UPF consumption and sleep has been the subject of several cross-sectional studies but has not been investigated prospectively. More than half of the reviewed studies (53%) were carried out in Brazil where the NOVA classification originated and where it has been the subject of a substantial body of research. In fact, Brazil is among more than a dozen countries that have addressed food processing in their national dietary guidelines [46].

Among the 13 studies that examined UPF consumption as the exposure and sleep as an outcome, 7 reported null findings and 6 observed significant associations. The latter reported that increased UPF consumption among adults was inversely associated with sleep problems, with the risk estimates ranging from 1.14 to 2.51 [30, 35, 36, 38]. The associations were consistent across sex [30, 33] and are congruent with findings among adolescents [47]. Of note, significant inverse associations were reported by the two studies conducted during the COVID-19 pandemic [36, 39] which has been independently associated with reduced sleep quality [48] and increased UPF consumption, especially during the lockdown periods [49].

Mechanistically, the impact of diet on sleep has been attributed to neuroendocrine regulation (e.g., serotonin, orexin, noradrenaline, histamine) and neuro-inflammatory processes that alter brain functionality via the gut-brain axis [26]. Indeed, the correct functioning of the sleep-wake cycle is promoted by melatonin, which is exclusively synthesized from dietary tryptophan, via serotonin [25]. Various dietary sources of tryptophan, serotonin, and melatonin, such as dairy, fish, fruit, and vegetables have been shown to have sleep-promoting effects [25, 50]. These foods are generally categorized as Groups 1 or 3 in the NOVA classification system, falling outside of the UPF category, Group 4. Moreover, reviews of the epidemiological and mechanistic evidence have highlighted the central role of the intestinal microbiota in connecting UPF and health status via alterations in the composition and function of the microbiota which are involved in food digestion, metabolism, and maturation of host immunity [13, 16].

The significant association of UPF consumption with risk of sleep problems is consistent with the large body of scientific evidence regarding the deleterious impact of UPF on a wide range of physical and mental health outcomes [13, 16••, 17,18,19,20,21]. To our knowledge, only one literature review with a meta-analysis has addressed the link between UPF and sleep [27]. These authors reviewed 15 cross-sectional studies carried out among children/adolescents (n = 8) and adults (n = 7), and modeling intake of any type of processed food as the exposure. Only two of the studies included in that review relied on NOVA for the assessment of UPF, with the other 13 studies using a variety of food groups (e.g., fast food, junk food, salty snacks, confectionary, soft drinks, etc.). The authors reported stronger effect sizes among the studies that used NOVA compared to those that used other processed food classifications. In addition, among adults, there were null findings for short sleep duration and statistically significant results for poor sleep quality [27].

Among the 7 studies that reported null findings in the present review, 1 was focused on menopausal symptoms, including sleep problems among 225 women aged ≥40 y [44]. A recent literature review evoked specific etiologic aspects of postmenopausal sleep difficulties, which might result from decreased estrogen and melatonin levels, vasomotor and psychological disturbances, and weight gain [51]. Next, 4 studies with non-significant findings pertained to baseline descriptions of cohorts used for the prospective investigation of chronic diseases [40,41,42,43]. For example, two of the studies, one regarding the incidence of obesity (n = 8,451) [42] and the other regarding the incidence of hypertension (n = 14,790) [43], were conducted within the Spanish SUN cohort by the same research team. In both studies, the sleep variable was a self-report of taking a nap (h/d), which did not vary significantly by the level of UPF consumption. It is possible that UPF intake differentially impacts daytime versus nighttime sleep parameters. Moreover, studies of napping included a single question related to this behavior and did not capture the intentionality of the nap. Future research ought to assess total sleep duration more thoroughly, including daytime and nighttime sleep, before conclusions could be drawn. Whereas De Oliveira et al. [29] did not find a significant association between UPF intake and sleep duration, assessed with a single question, they reported a significant correlation between low consumption of unprocessed or minimally processed food and inadequate sleep duration using a mixed adolescent-adult sample of 432 participants. Specifically, the median intake of such food among participants with adequate (defined as 8-10 h sleep/d) and inadequate sleep duration was 996.2 and 729.9 Kcal/d, respectively.

It is interesting to note that 5 of the 6 studies showing significant associations between UPF consumption and sleep assessed sleep quality whereas 6 of the 7 studies that reported null findings examined sleep duration (with 2 studies measuring solely napping duration from a single question). Thus, it is possible that diet quality is a more important modulator of sleep quality rather than sleep quantity. Future studies should extend beyond single questions of sleep duration and quality to truly capture overall sleep health, which includes concepts related to regularity, satisfaction, alertness, timing, efficiency, and duration [52]. In the present review, only one study [33] employed a comprehensive assessment of sleep with objective and subjective measures such as accelerometry-assessed sleep onset and offset, total sleep time and variability, sleep efficiency, along with PSQI and Epworth’s excessive daytime sleepiness [53]. In turn, Noll et al. [44] employed a composite sleep index based on both the Women's Health Questionnaire, where 2 of the 37 items pertain to sleep (insomnia underscored by difficulty falling asleep, early waking and restlessness) [54], and the Kupperman-Blatt Menopausal Index where insomnia is one of 11 types of menopausal symptoms that are all grouped together [55].

Two of the 15 studies included in this review investigated sleep as the exposure and UPF consumption as the outcome of interest. Both studies reported significant inverse associations between sleep duration [45] or quality [31] and UPF consumption in adjusted analyses. This is in line with the large body of observational and experimental evidence regarding the impact of sleep on dietary choices and diet quality and quantity [25]. For example, poor sleep quality has been associated with higher intakes of energy, sugar and fat, and lower intakes of fruit, vegetables, and whole grains [56,57,58]. Such dietary behaviors have been explained by a combination of factors, including an increased opportunity to eat due to added wake time, altered time of intake (i.e., late evening), changes in hormonal regulation, in reward valuation, and in taste sensitivity, as well as a potential homeostatic compensation effect for nocturnal energy deficit, and increased susceptibility to food stimuli [25, 26, 59]. The current review adds to the available knowledge by expanding the range of dietary outcomes influenced by sleep disturbances. Mechanistically, an increased hedonic drive for foods might explain shifts toward poor diet quality following periods of inadequate sleep [25]. Indeed, sleep restriction has been shown to result in increased hunger, appetite, and energy intake [60]. In turn, UPF exposure has been positively associated with appetitive drive and hedonic valence [61].

This review revealed substantial heterogeneity in the reporting of UPF intake, which included number of servings per day or per week, grams per day, Kcal per day, and percentage contribution of UPF to mean daily energy intake. Thus, differences among the reviewed studies might be partly due to different UPF amounts consumed within each NOVA category. This methodological heterogeneity might also help explain the differences between the studies showing significant associations and those reporting null findings. Dietary data for the studies came from various sources, including standard or semi-quantitative FFQ (9 studies), a NOVA-specific questionnaire (2 studies), and 24-h recalls (4 studies). It has been suggested that the use of multiple 24-h dietary records is preferrable over FFQ for UPF estimation [16]. In the present review, however, only three studies relied on more than one 24-h recall [40, 41, 44], finding no association between UPF consumption and menopause-related insomnia symptoms or sleep duration. Methodological heterogeneity was also observed in the self-reported sleep measures, which included average sleep duration, sleep quality, afternoon napping, and anxiety-induced insomnia/sleep disturbance, with more than half of these studies relying on a single question for the assessment of sleep. Finally, in the case of anxiety-induced sleep problems, the measure pertained to “insomnia due to worries or concerns” rather than an actual anxiety assessment or diagnosis [30].

Evidence regarding the bidirectional association between diet and sleep has been evaluated and summarized in several systematic reviews. For example, Sutanto et al. [62] reviewed observational and intervention studies regarding the role of the dietary macronutrient composition in sleep duration and reported inconclusive findings without a dose-dependent association. These authors concluded that the macronutrient profile alone may not play a strong role in sleep [62]. Next, evidence for the role of specific food groups, micro- and macro-nutrients, and dietary patterns in sleep quality among children, adolescents, and adults was recently reviewed by Godos et al. [26]. Whereas the methodological quality of the included studies was not high according to the NIH quality assessment measure, the authors were able to conclude that for some aspects of the diet (e.g., protein, carbohydrate content), the type and quality might be more important than the quantity of intake [26•].

An increased dietary share of UPF has been associated in a direct, dose-response manner with the dietary content of free/added sugars, saturated fat, trans fat, sodium, and energy density, whereas an inverse dose-response association has been found with protein, fiber, potassium, vitamins A, C, D, and E, calcium, zinc, magnesium, phosphorus [63•] and water intake [64]. Higher UPF intakes have been inversely associated with intake of fruit, vegetables, legumes, and seafood [65] which are all sources of sleep-promoting compounds. Indeed, prospective research has shown that individuals adhering to nutrient-dense and fiber-rich diets, such as the Mediterranean diet, have better sleep health [66] and lower risk of insomnia [67]. In contrast, higher dietary glycemic index and glycemic load, underscored by an increased intake of added sugars, starch, and refined grains, have been suggested as independent risk factors for insomnia incidence [68]. Likewise, pooled fully adjusted analyses of three large US cohorts demonstrated that a higher dietary inflammatory potential was associated with a significantly greater risk of obstructive sleep apnea [69].

The present systematic review is subject to some limitations. It cannot provide any evidence of causal effects as it included only cross-sectional research. It was based on published observational studies, none of which employed non-linear modelling. Another important limitation pertains to the inconsistent adjustment for potential confounders of the UPF-sleep association. The review identified notable methodological weaknesses related to the assessment of diet and sleep. It is recommended that future research in this domain employ validated tools and established criteria for the assessment of sleep, such as the DSM-5 criteria for chronic insomnia [70], the Berlin questionnaire for obstructive sleep apnea [71], Epworth Sleepiness Scale [53] (used in only one of the reviewed studies), and the PSQI [34] (used in less than a third of the reviewed studies). Objective measures of sleep (also used in only one of the reviewed studies) including duration, wake after sleep onset, and sleep efficiency are likewise needed. In addition, future epidemiological research conducted outside of Brazil could augment the generalizability of the overall evidence.